U.S. patent number 7,951,728 [Application Number 11/860,161] was granted by the patent office on 2011-05-31 for method of improving oxide growth rate of selective oxidation processes.
This patent grant is currently assigned to Applied Materials, Inc.. Invention is credited to Balasubramanian Ramachandran, Martin John Ripley, Norman Tam, Yoshitaka Yokota.
United States Patent |
7,951,728 |
Yokota , et al. |
May 31, 2011 |
Method of improving oxide growth rate of selective oxidation
processes
Abstract
A method for selective oxidation of silicon containing materials
in a semiconductor device is disclosed and claimed. In one aspect,
a rapid thermal processing apparatus is used to selectively oxidize
a substrate by in-situ steam generation at high pressure in a
hydrogen rich atmosphere. Other materials, such as metals and
barrier layers, in the substrate are not oxidized.
Inventors: |
Yokota; Yoshitaka (San Jose,
CA), Tam; Norman (San Jose, CA), Ramachandran;
Balasubramanian (Santa Clara, CA), Ripley; Martin John
(San Jose, CA) |
Assignee: |
Applied Materials, Inc. (Santa
Clara, CA)
|
Family
ID: |
40297951 |
Appl.
No.: |
11/860,161 |
Filed: |
September 24, 2007 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20090081884 A1 |
Mar 26, 2009 |
|
Current U.S.
Class: |
438/773; 438/765;
438/781; 257/E21.639; 257/E21.285; 257/E21.654; 257/E21.282;
257/E21.268; 438/799; 438/769; 438/787 |
Current CPC
Class: |
H01L
21/31662 (20130101); H01L 21/28247 (20130101); H01L
21/32 (20130101); H01L 21/02255 (20130101); H01L
21/0223 (20130101); H01L 21/02238 (20130101) |
Current International
Class: |
H01L
21/00 (20060101) |
Field of
Search: |
;438/585,592,653,689,694,700,765-787,799,935
;257/E21.002,E21.029,245,283,285,301,703,E21.268-E21.285,639,654,E29.132 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
Ohnishi et al., Improving gate oxide integrity (GOI) of a
W/WNx/dual-poly Si stacked-gate by using Wet- Hydrogen oxidation in
0.14-.mu.m CMOS devices, IEEE 397-400 (Sep. 1998). cited by other
.
Lin, B. et al., "Selective Oxidation of SI in the Presence of W and
WN" Materials Research Society Symposium Proceedings, Materials
Research Society, Pittsburg, PA, vol. 525, Apr. 1998, pp. 359-364,
XP000879331. cited by other .
Joo, H.S. et al. "A Novel Method for Selective Oxidation of
W/WNx/Poly Gate Stack Using RTP ISSG Technique" Electromechanical
Society Proceedings, No. 99-18, 1999, pp. 203-209, XP001019637.
cited by other .
Hiura, Y., et al., "Integration Technology of Polymetal
(W/WSiN/Poly-Si) Dual Gate CMOS for 1 Gigabit DRAMs and Beyond",
Toshiba Corporation, IEDM 98-389. cited by other .
Shah, Nitin and Rahul Sharangpani, "Selective Oxidation by
Hydrogen-Rich Steam", European Semiconductor Nov., 1999. cited by
other .
Roters, Georg, et al., "Selective Oxidation of Tungsten-Gate Stacks
in High Volume DRAM Production", Proceedings, 203rd Meeting of the
Electrochemical Society, Paris (2003). cited by other .
Liu, Yong, et al., "Selective Oxidation of Silicon (100) vs.
Tungsten Surfaces by Steam in Hydrogen", Journal of the
Electrochemical Society, 150 (10), G597-G601 (2003). cited by other
.
Kee, Robert, J., et al., "The Influence of Pressure, Fluid Flow,
and Chemistry on the Combustion-Based Oxidation of Silicon", 28
Proc. Combust. Inst. 1381-1388 (2000). cited by other .
Table A2.2, NASA NSS 1740.16 (1997). cited by other .
Akasaka, Y., et al. Conference Proceedings ULSI XIV p. 613,
Materials Research Society (1999). cited by other.
|
Primary Examiner: Lebentritt; Michael S
Attorney, Agent or Firm: Patterson & Sheridan,
L.L.P.
Claims
The invention claimed is:
1. A method of selectively oxidizing materials of a composite
substrate, comprising: disposing the composite substrate in a
chamber; introducing a gas mixture to the chamber, the gas mixture
comprising an oxygen containing gas and a hydrogen containing gas,
wherein the hydrogen containing gas is about 85% of the gas
mixture; pressurizing the chamber to about 450 torr; and heating
the chamber to a predetermined temperature for a predetermined time
causing the hydrogen containing gas and the oxygen containing gas
to react inside the chamber, selectively oxidizing the composite
substrate.
2. The method of claim 1, wherein the predetermined temperature is
greater than about 700.degree. C.
3. The method of claim 1, wherein the hydrogen containing gas and
the oxygen containing gas are mixed outside the chamber to produce
the gas mixture.
4. The method of claim 2, wherein selectively oxidizing the
composite substrate comprises oxidizing only silicon containing
materials.
5. The method of claim 4, wherein the silicon containing materials
comprise silicon, doped silicon, polysilicon, doped polysilicon,
amorphous silicon, doped amorphous silicon, microcrystalline
silicon, doped microcrystalline silicon, silicon dioxide
(SiO.sub.2), or combinations thereof.
6. The method of claim 1, wherein the composite substrate comprises
a substrate having one or more layers of silicon-containing
material and one or more layers of metal-containing material.
7. A method of processing a substrate, comprising: disposing the
substrate in a rapid thermal processing (RTP) chamber; introducing
a non-reactive gas to the chamber; introducing an amount of a
hydrogen containing gas and an amount of an oxygen containing gas
to the chamber to form a gas mixture, wherein the gas mixture
comprises a hydrogen rich gas mixture; pressurizing the chamber to
a pressure greater than about 250 torr; heating the chamber to a
processing temperature to cause the gas mixture to react inside the
chamber; and selectively oxidizing the substrate.
8. The method of claim 7, wherein the amount of the hydrogen
containing gas is between about 70% and about 90% of the amount of
the hydrogen rich gas mixture.
9. The method of claim 7, further comprising controlling the amount
of the oxygen containing gas by specifying a ratio to the amount of
the hydrogen containing gas.
10. The method of claim 7, wherein the substrate comprises at least
a silicon containing layer and a metal layer.
11. The method of claim 10, wherein the silicon containing layer
comprises silicon, doped silicon, polysilicon, doped polysilicon,
amorphous silicon, doped amorphous silicon, microcrystalline
silicon, doped microcrystalline silicon, silicon dioxide
(SiO.sub.2), or combinations thereof.
12. The method of claim 10, wherein the selectively oxidizing the
substrate comprises oxidizing only the silicon containing
layer.
13. The method of claim 7, wherein the amount of the hydrogen
containing gas is about 85% of the amount of the hydrogen rich gas
and the pressure is about 450 torr.
14. A method of processing a substrate, comprising at least a
silicon containing layer and a metal layer, in a chamber,
comprising: introducing a hydrogen rich gas mixture to the chamber;
pressurizing the chamber to a pressure greater than about 250 torr;
reacting the hydrogen rich gas mixture inside the chamber to
produce steam; and selectively oxidizing the silicon containing
layer.
15. The method of claim 14, wherein the hydrogen rich gas comprises
greater than about 65% hydrogen gas (H.sub.2), a non-reactive gas,
and an oxygen containing gas, wherein the non-reactive gas is
selected from the group consisting of helium (He), nitrogen gas
(N.sub.2), argon (Ar), neon (Ne), xenon (Xe), and combinations
thereof.
16. The method of claim 14, wherein the silicon containing layer
comprises silicon, doped silicon, polysilicon, doped polysilicon,
amorphous silicon, doped amorphous silicon, microcrystalline
silicon, doped microcrystalline silicon, silicon dioxide
(SiO.sub.2), or combinations thereof.
17. The method of claim 14, further comprising producing an oxide
layer with a thickness less than about 100 Angstroms.
18. The method of claim 14, wherein a quantity of the hydrogen gas
(H.sub.2) is controlled in proportion to a quantity of the hydrogen
rich gas.
19. The method of claim 14, wherein the metal layer comprises
tungsten (W), tungsten nitride (WN), titanium (Ti), titanium
nitride (TiN), tantalum (Ta), tantalum nitride (TaN), cobalt (Co),
or combinations thereof.
20. The method of claim 18, wherein the quantity of the hydrogen
gas (H.sub.2) is about 85% of the quantity of the hydrogen rich gas
and the pressure is about 450 torr.
21. The method of claim 14, further comprising heating the chamber
to a predetermined temperature.
22. The method of claim 21, wherein the hydrogen rich gas mixture
is introduced after the chamber is heated to the predetermined
temperature.
23. A method of processing a substrate comprising one or more oxide
layers and one or more metal or barrier layers, comprising:
disposing the substrate in a chamber; introducing a quantity of a
hydrogen containing gas and a quantity of an oxygen containing gas
into the chamber to create a quantity of a gas mixture, wherein the
quantity of the hydrogen containing gas is about 65% to about 85%
of the quantity of the gas mixture; pressurizing the chamber to a
pressure greater than about 250 torr; reacting the hydrogen
containing gas and the oxygen containing gas inside the chamber to
produce steam; and oxidizing only the one or more oxide layers on
the substrate.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
Embodiments of the present invention generally relate to the field
of semiconductor manufacturing and more specifically to a method
and apparatus for the selective oxidation of a composite
silicon/metal film.
2. Description of the Related Art
In the manufacture of semiconductor devices, oxidation of silicon
containing substrates plays a key role. For example, in a standard
semiconductor device, a gate oxide layer is ordinarily situated
over a substrate containing a source region, a drain region, and an
intervening silicon or polysilicon region. Metal contacts are
deposited over the source and drain regions, and a conductive layer
deposited over the gate oxide. The entire structure is often
depicted as a stack of layers. When a voltage is applied across the
gate oxide generating an electric field oriented along an axis from
the substrate, through the gate oxide, to the conductive layer, the
electrical characteristics of the region between the source and
drain region change, either allowing or stopping the flow of
electrons between the regions. The gate oxide layer thus occupies a
pivotal role in the structure of semiconductor devices.
Often, properties of the device are improved by deposition of other
layers in the device. For example, to control diffusion of metal
atoms into the gate oxide layer, which degrades the dielectric
properties of the gate oxide, a barrier layer may be deposited
between the gate oxide and the metal layer. Also, a hard mask layer
may be deposited over the metal layer. In order to promote adhesion
of such layers, smooth their surfaces, and harden them to
diffusion, the barrier or hard mask layers may be treated with a
plasma. The plasma treatment can degrade the properties of the gate
oxide layer by eroding it from the sides or reducing its thickness.
Likewise, the gate oxide layer may be damaged by repeated cycles of
deposition, etching, and plasma processing typically involved in
modern device fabrication. This damage degrades the gate
characteristics of the layer, rendering the device inoperative.
To repair the damage to the oxide layer, it is possible to
re-oxidize the device. Re-oxidation creates a thin layer of oxide
on the sides of the gate oxide and underlying silicon containing
layers, repairing the edge damage. Because oxidizing other regions
of the transistor may reduce conductivity and impair the device,
oxidizing only certain materials in the device is desired. For
example, oxidizing the metal cap over the gate and the metal
contacts over the source and drain regions reduces their
conductivity. Likewise, a given device may contain more than just
the metal surfaces associated with transistors. Selective oxidation
targets certain materials, such as silicon and oxides of silicon,
while avoiding oxidation of other materials.
Conventional oxygen rich processes oxidize not only the desired
layers, but also undesired layers such as metals and barrier
layers. Wet oxidation processes, although faster than dry
processes, do not promote oxide growth as quickly as steam
oxidation. FIGS. 1A-1C set forth oxidation rates of silicon for dry
oxidation, wet oxidation, and steam oxidation, respectively.
Subjecting a device to heat under an atmosphere of dilute steam
rich in hydrogen gas (H.sub.2) at low pressure can selectively
oxidize silicon containing materials without oxidizing metals or
barrier layers. As can readily be appreciated, however, operating a
hydrogen combustion chamber at high temperature and pressure has
heretofore required combustion of hydrogen in a separate location.
At higher pressures, and with long soak times, hydrogen gas may
attack barrier and hard mask layers, reducing their effectiveness
and forming unwanted metal silicide layers with higher
resistivity.
Thus, there is still a need for a selective oxidation process
utilizing in situ steam generation that efficiently oxidizes only
silicon containing layers of a semiconductor device stack without
degrading the properties of barrier or conductive layers.
SUMMARY OF THE INVENTION
The present invention generally provides a method of selectively
oxidizing silicon containing materials of a composite substrate,
comprising disposing the composite substrate in a chamber,
introducing a gas mixture comprising an oxygen containing gas and a
hydrogen containing gas to the chamber, such that the ratio of the
hydrogen containing gas to the gas mixture is greater than about
65%, pressurizing the chamber to a pressure between about 250 torr
and about 800 torr, and heating the chamber to a predetermined
temperature for a predetermined time to cause the hydrogen
containing gas and the oxygen contain gas to react inside the
chamber, selectively oxidizing the composite substrate.
Some embodiments of the invention include a method of selectively
oxidizing materials of a composite substrate, comprising disposing
the composite substrate in a chamber, introducing a gas mixture to
the chamber, the gas mixture comprising an oxygen containing gas
and a hydrogen containing gas, and an amount of the hydrogen
containing gas is greater than about 65% of an amount of the gas
mixture, pressurizing the chamber to a pressure between about 250
torr and about 800 torr, and heating the chamber to a predetermined
temperature for a predetermined time causing the hydrogen
containing gas and the oxygen contain gas to react inside the
chamber, selectively oxidizing the composite substrate.
Other embodiments of the invention provide a method of processing a
substrate, comprising disposing the substrate in a rapid thermal
processing (RTP) chamber, introducing an amount of a hydrogen
containing gas and an amount of an oxygen containing gas to the
chamber to form a gas mixture, wherein the gas mixture comprises a
hydrogen rich gas mixture, pressurizing the chamber to a pressure
greater than about 250 torr, heating the chamber to a processing
temperature to cause the gas mixture to react inside the chamber,
and selectively oxidizing the substrate.
Other embodiments of the invention provide a method of processing a
substrate, comprising at least a silicon containing layer and a
metal layer, in a chamber, comprising introducing a hydrogen rich
gas mixture to the chamber, pressurizing the chamber to a pressure
greater than about 250 torr, reacting the hydrogen rich gas mixture
inside the chamber to produce steam, and selectively oxidizing the
silicon containing layer.
Further embodiments of the invention provide a method of processing
a substrate comprising one or more oxide layers and one or more
metal or barrier layers, comprising disposing the substrate in a
chamber, introducing a quantity of a hydrogen containing gas and a
quantity of an oxygen containing gas into the chamber to create a
quantity of a gas mixture, wherein the quantity of the hydrogen
containing gas is about 65% to about 85% of the quantity of the gas
mixture, pressurizing the chamber to a pressure greater than about
250 torr, reacting the hydrogen containing gas and the oxygen
containing gas inside the chamber to produce steam, and oxidizing
only the one or more oxide layers on the substrate.
BRIEF DESCRIPTION OF THE DRAWINGS
So that the manner in which the above recited features of the
present invention can be understood in detail, a more particular
description of the invention, briefly summarized above, may be had
by reference to embodiments, some of which are illustrated in the
appended drawings. It is to be noted, however, that the appended
drawings illustrate only typical embodiments of this invention and
are therefore not to be considered limiting of its scope, for the
invention may admit to other equally effective embodiments.
FIG. 1A is a graph of oxidation rate of silicon under dry
conditions.
FIG. 1B is a graph of oxidation rate of silicon under wet
conditions.
FIG. 1C is a graph of oxidation rate of silicon under steam
conditions.
FIG. 2 is an illustration of a rapid thermal heating apparatus that
can be used with the process of the present invention.
FIG. 3A is a flowchart that illustrates one embodiment of the
present invention.
FIG. 3B is a flowchart that illustrates another embodiment of the
present invention.
FIG. 3C is a flowchart that illustrates another embodiment of the
present invention.
FIG. 4A is a cross-sectional view of a substrate prior to applying
a selective oxidation process according to one embodiment of the
present invention.
FIG. 4B is a cross-sectional view of a substrate following
application of a selective oxidation process according to one
embodiment of the present invention.
FIG. 5 is a graph showing concentration of reactive species at
different pressures.
FIG. 6 is a process window diagram illustrating operable process
conditions according to one embodiment of the invention.
DETAILED DESCRIPTION
The present invention describes a method for selectively oxidizing
silicon containing materials in a substrate. While the invention
will be described below in reference to a rapid thermal heating
chamber, such as any of the VANTAGE.TM. or CENTURA.TM. devices
available from Applied Materials, Inc., of Santa Clara, Calif., it
is to be understood that the invention may be practiced in other
chambers, including those sold by other manufacturers. FIG. 2
illustrates a rapid thermal heating apparatus 200 that can be used
to carry out the process of the present invention. The apparatus
features a process chamber 202 that may be evacuated or filled with
selected gases, and a side wall 204 and bottom enclosure 206. The
upper portion of the sidewall is sealed against a light pipe
assembly 208, from which radiant energy is directed into the
chamber. The light pipe assembly 208 includes a plurality of
tungsten halogen lamps 210, for example Sylvania EYT lamps, each
mounted into a light pipe 212 which may be made of stainless steel,
brass, aluminum, or other metal.
A substrate 214 is supported within the process chamber 200 by a
support ring 216 that contacts the edge of the substrate. The
support ring 216 is made of a material capable of withstanding high
temperatures, such as silicon carbide, without imparting impurities
to the substrate. The support ring 216 may be mounted on a rotation
cylinder 218. In one embodiment, a quartz rotation cylinder capable
of rotating the support ring and substrate thereon may be used.
Rotation of the substrate promotes uniform temperature
distribution.
Process gases may be admitted to the chamber through representative
portal 220, and exhaust evacuated through representative portal
222. In some embodiments, multiple gas feed and exhaust portals may
be used. A temperature controller 224 receives measurements from
pyrometers 226 and adjusts power to lamps 210 to achieve even
heating.
FIG. 3A is a flowchart that illustrates a method of selectively
oxidizing a substrate according to the present invention. The first
step in the process 310 is to purge any reactive gases from the
chamber. Purging avoids any unwanted chemical reactions on the
substrate during preparatory phases of the oxidation treatment when
temperatures and pressures may be elevated. It is an objective of
the present invention to oxidize only silicon containing layers of
a composite substrate comprising silicon containing layers, metal
layers, and optionally barrier or capping layers. To accomplish
this objective, composition of gases in the process chamber may be
controlled during any process step featuring elevated temperature
or pressure. The purge is accomplished by pumping all gases out of
the chamber and then flowing a non-reactive gas into the chamber to
create a non-reactive gas atmosphere in the process chamber. The
non-reactive gas does not react with any substrate material during
processing. Gases which are non-reactive in the process of the
present invention include, but are not limited to, nitrogen gas
(N.sub.2), helium (He), argon (Ar), neon (Ne), and xenon (Xe).
A substrate having multiple layers of silicon containing materials,
metals, and optionally barrier or capping layers is disposed within
the chamber in the next step of the process 312. The layers may be
patterned to form device structures, such as transistors, on the
substrate. FIG. 4A illustrates a typical gate transistor structure
400. Doped silicide regions 402 are disposed within a polysilicon
domain 404 of the substrate. The doped silicide regions 402 form
source and drain regions for the transistor. Over the doped
silicide regions 402, multiple layers of polysilicon 406, gate
oxide 408, barrier material 410, metal contacts 412, and protective
or hard mask material 414 may be deposited. Additionally, and not
shown, metal contacts may be deposited directly atop the doped
silicide regions, with or without barrier or nucleation layers
between. The process of the present invention selectively oxidizes
only the polysilicon and gate oxide layers, along with other
silicon containing areas of the substrate, without oxidizing the
metal or other layers.
The substrate may be introduced to the chamber through a slit valve
in the process chamber. A transfer robot configured as part of a
processing cluster or platform may be used to load the substrate
into the chamber. Alternately, a tray loader may be used with a
cartridge device to load and unload multiple substrates
consecutively. Furthermore, a carousel arrangement may be used to
transport substrates into and out of the process chamber as part of
a rotary processing cluster, or a linear processing assembly may be
used.
Referring once again to FIG. 3A, the substrate supported on the
support ring in the process chamber under a non-reactive atmosphere
is next subjected to a temperature and pressure ramp-up step 314.
Hydrogen containing gas may be fed to the process chamber prior to
ramping-up temperature and pressure. Alternately, the non-reactive
atmosphere may be maintained during the ramp-up by flowing
non-reactive gas into and out of the process chamber. Pressure in
the chamber may be precisely controlled and any fugitive emissions
that may escape the substrate removed by the flowing gases as
temperature increases. Temperature and pressure may be ramped in
any pattern, simultaneously or consecutively, up to the desired
predetermined process conditions. The temperature ramp may be
designed to confer the added benefit of annealing any of the
various layers of the substrate. It has been found that the best
selective oxidation conditions are obtained at pressures between
about 150 torr to about 800 torr, especially between about 250 torr
to 600 torr, such as 450 torr. It has also been found that the best
selective oxidation conditions are achieved at temperatures greater
than about 700.degree. C., especially between about 800.degree. C.
and about 1000.degree. C., such as about 950.degree. C.
Referring again to FIG. 3A, a hydrogen containing gas may be fed to
the process chamber before or after ramping-up temperature and
pressure. Although hydrogen (H.sub.2) gas is preferred, another gas
capable of producing water vapor when oxidized, such as ammonia
(NH.sub.3) may be used. When the desired flow rate of hydrogen
containing gas is reached, and operating conditions established, an
oxygen containing gas is fed into the process chamber to create a
gas mixture in step 318. Although oxygen (O.sub.2) gas is
preferred, other oxidizing gases, such as nitrous oxide (N.sub.2O)
may be used. The flow rate of the oxygen containing gas is ramped
up to a set point to allow temperature, pressure, and flow controls
to respond as the reaction begins. The hydrogen containing gas and
the oxygen containing gas react, generating in-situ steam, which in
turn drives the selective oxidation reaction on the substrate. It
is thought that water molecules diffuse into the silicon containing
material crystal network, liberating hydrogen at Si--Si or
Si--SiO.sub.2 bonds. Processing continues in step 320 until a
predetermined end point is reached, such as a certain amount of
time. Temperature is reduced and the chamber evacuated in step 322
to remove reactive species. A non-reactive gas is once again
charged to the chamber in step 324 to complete the process, after
which the substrate is removed in step 326.
FIGS. 3B and 3C illustrate alternate embodiments of the process of
the invention. FIG. 3B illustrates a variation in which a
non-reactive gas is used to establish chamber operating pressure in
step 334 before hydrogen containing gas is added in step 336.
Temperature may be established after charging hydrogen containing
gas, as shown in step 338, and then oxygen containing gas charged
to the chamber in step 340. FIG. 3C illustrates a variation in
which the oxygen containing gas is charged first in step 354,
followed by pressuring up in step 356, charging hydrogen containing
gas in step 358, and establishing temperature in step 360.
In alternate embodiments, hydrogen containing gas and oxygen
containing gas may both be ramped-up after the chamber reaches the
desired temperature and pressure, with the advantage that a single
flow upset will not result in unwanted acceleration of the
reaction. In another alternate embodiment, hydrogen containing gas
may be introduced to the chamber before reaching the desired
temperature and pressure points, with the potential advantage of
passivating any metal layers on the substrate, further reducing the
oxidation potential of the metals. In other embodiments, a
non-reactive or carrier gas may be used with the hydrogen
containing gas or the oxygen containing gas, or both, and may be
fed separately or with either gas. The gases may be mixed outside
the reaction chamber or fed individually to the chamber. Use of a
non-reactive gas may promote mixing and selectivity, but will
likely reduce oxidation rate.
The reaction is driven by the temperature and pressure in the
reaction zone. The reaction zone is heated by convection from the
hot substrate and by energy released from the oxidation reaction.
Temperatures required to drive the reaction are thus found in the
immediate vicinity of the substrate surface. In some embodiments,
the reaction may be confined to a zone up to 1 cm from the
substrate surface. Temperatures above 700.degree. C. are generally
effective to promote selective oxidation reactions. Temperature is
controlled through sensors disposed in the chamber and connected to
a temperature controller that varies power to the heat lamps.
Effective control of flow rates, temperatures, and pressures
contributes to a successful selective oxidation process. If there
is too much oxygen in the gas mixture, oxygen radical species
predominate, causing unwanted oxidation reactions. FIG. 5 is a
reactive species diagram showing relative concentration of
oxidizing species at different pressures for a gas mixture
characteristic of traditional oxygen rich oxidation reactions. It
illustrates the general rule that as pressure increases,
concentration of undesirable oxidizing species declines. Due to
their size, oxygen radical species are better able to diffuse into
the crystal structure of metals than are water molecules. Thus,
higher concentration of oxygen radical species results in lower
selectivity for silicon containing materials. Higher chamber
pressures result in fewer radical species because oxygen radicals
are quickly scavenged by hydrogen containing species.
While it is desirable to maximize the rate of the selective
oxidation reaction, oxidation and combustion reactions can be
explosive if the wrong mixture of reactants is used. It has been
found that a mixture of hydrogen gas (H.sub.2) and oxygen gas
(O.sub.2) of greater than about 65% hydrogen creates the most
advantageous reaction conditions. A hydrogen rich gas mixture
generally results in acceptable oxidation rate and high
selectivity. FIG. 6 shows a reaction rate diagram relating to the
process of the present invention. Region 1 represents operating
windows used currently in selective oxidation processes. Region 2
represents the large explosive potential of mixtures of hydrogen
and oxygen gas, compositions which must be avoided. Region 3
represents the target operating window of the present invention.
Favorable results have been obtained with a mixture of about 65% to
about 95% hydrogen in oxygen, especially about 75% to about 90%,
such as about 85% hydrogen in oxygen. At these conditions, slight
variations in composition within the reaction chamber can lead to
substantial temperature variation. Likewise, slight variation in
flow rate of reactants can cause the reaction mixture to approach
explosive limits. Interlocks are used to ensure the flow rate of
oxygen containing gas stays below control limits with an acceptable
margin of safety. The amount of oxygen containing gas fed to the
reactor may be controlled by specifying a ratio of oxygen
containing gas flow rate to hydrogen containing gas flow rate, or
by specifying ratios of both to a non-reactive or carrier gas, or
by any other method designed to control the proportion of reactive
gases in the gas mixture.
The reaction is allowed to proceed a set amount of time. A thin
film of oxide growth on the silicon containing materials of the
substrate is desired. At these process conditions, a duration of
about 1 to about 5 minutes is sufficient to produce a new oxide
layer 20 to 50 Angstroms thick. FIG. 4B illustrates a device
structure 420 after selective oxidation has been performed. Oxide
layers 416 have grown adjacent to silicon containing layers of the
structure. With the process of the present invention, oxidation
selectivities of polysilicon and silicon dioxide relative to
tungsten metal up to 99.6% have been obtained. When the end point
is reached, temperature may be ramped down and the reaction chamber
may be pumped out and non-reactive gas charged. The chamber may be
purged briefly to ensure no potentially reactive gases remain to
degrade the substrate, and then the substrate is removed from the
chamber for further processing.
The foregoing process may be used to selectively oxidize many
silicon containing materials on a substrate. Such silicon
containing materials include, but are not limited to, polysilicon
(or polycrystalline silicon), doped silicon, microcrystalline
silicon, doped microcrystalline silicon, amorphous silicon, doped
amorphous silicon, generic silicon, doped or undoped, not fitting
any of the former labels, partially oxidized silicon materials
substantially comprising silicon dioxide (SiO.sub.2), and
combinations thereof. Likewise, many popular metal conductors and
barrier or protective layers may be safely exposed to this process.
Metal layer compositions which will not be oxidized under such
conditions include, but are not limited to, aluminum (Al), copper
(Cu), tungsten (W), tungsten nitride (WN), titanium (Ti), titanium
nitride (TiN), tantalum (Ta), tantalum nitride (TaN), tantalum
carbonitride (TaCN), and combinations thereof.
Examples of selective oxidation treatments performed according to
embodiments of the invention are set forth in Table 1 below. The
embodiments below illustrate selective oxidation of silicon versus
tungsten metal using hydrogen gas and oxygen gas as the reactive
species. As mentioned above, however, it should be understood that
other hydrogen containing gases, such as ammonia, may be used, and
other oxygen containing gases, such as nitrous oxide, may be used
to perform processes representing other embodiments of the
invention. As also mentioned above, embodiments in accordance with
this invention other than those specifically illustrated in Table 1
will achieve selective oxidation relative to metals other than
tungsten.
TABLE-US-00001 TABLE 1 Percent Temperature Hydrogen in Example (C.)
Pressure (torr) Oxygen Selectivity (%) 1 850 150 85 3 2 850 300 95
99.4 3 850 450 85 98.4 4 950 150 85 5 5 950 300 95 99.3 6 950 450
85 99.6
CONCLUSION
Embodiments of the present invention relating to a method and
apparatus for the selective oxidation of a composite silicon/metal
film have been described. Silicon containing materials in a
semiconductor device are oxidized quickly without oxidizing other
layers of the device, such as metal layers, through high pressure
in-situ steam generation. While the foregoing is directed to
embodiments of the present invention, other and further embodiments
of the invention may be devised without departing from the basic
scope thereof. The scope of the invention is defined by the claims
that follow.
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